1. Field of the Invention
The present invention relates to a reversible thermosensitive recording medium in which an image can be reversibly recorded and erased, and a reversible thermosensitive recording device using the thermosensitive recording medium. More particularly, the present invention relates to a reversible thermosensitive recording medium which achieves a relatively colored state or a relatively discolored state utilizing at least one of differences in heating temperature and cooling speed after heating. In addition, the present invention also relates to an image processing method and apparatus using the thermosensitive recording medium.
2. Discussion of the Related Art
Recently, reversible thermosensitive recording media in which an image is recorded, and the recorded image can be erased if desired, attract attention. Among the reversible thermosensitive recording media, a medium in which a color developer, such as organic phosphoric acid compounds, aliphatic carboxylic acid compounds and phenolic compounds, which have a long aliphatic hydrocarbon chain, and a coloring agent such as leuco dyes are dispersed in a resin is well known.
For example, published unexamined Japanese Patent Applications Nos. (hereinafter referred to as JP-A) 10-67177 and 10-119440 have disclosed reversible thermosensitive coloring compositions including an electron accepting compound and a phenolic compound. These reversible thermosensitive coloring compositions have such advantages as to be able to record high-contrast images and erase images at a high speed. In addition, the compositions also have an advantage such that recorded images can be erased with a thermal printhead under normal temperature and normal humidity conditions, and have a potentiality such that overwriting is performed using a thermal printhead. However, the compositions have a drawback in that recorded images cannot be well erased under low temperature and low humidity conditions.
JP-A 08-108627 discloses a reversible thermosensitive recording material in which one or more of adducts of ethylene oxide, propylene oxide or butylene oxide are used as an auxiliary erasing agent. JP-A 08-108627 did not disclose specific information about the molecular weight of the adducts, and an adduct of polyethylene oxide having an average molecular weight less than 2,000 is used for examples of the recording material. In addition, there is no description or suggestion in JP-A 08-108627 as to whether the recording material have an improved erasing property under low temperature/low humidity conditions.
JP-A 08-085255 (i.e., Japanese Patent No. 3,075,101) discloses a reversible thermosensitive recording material using a compound having a polyoxyethylene chain in the molecule thereof as an auxiliary erasing agent. This recording material includes a developing/reducing reagent, which has both an acid group and a basic group in its molecule, an essential material. Namely, the developing controlling agent releases a hydrogen ion when heated to a relatively high temperature, and serves as a base to erase recorded images when heated to a relatively low temperature. This image recording/erasing mechanism is different from the typical reversible thermal image recording/erasing mechanism in which a recording material achieves a colored state or a discolored state utilizing at least one of differences in heating temperature and cooling speed after heating. In addition, JP-A 08-085255 does not disclose information about number average molecular weight of the compounds having a polyoxyethylene chain. Further, JP-A 08-085255 does not disclose or suggest whether the recording material has an improved erasing property under low temperature and low humidity conditions.
Because of these reasons, a need exists for a reversible thermosensitive recording medium which can reversibly record an image having a high image density (i.e., a high contrast) and erase an image at a high speed even under various environmental conditions of from low temperature and low humidity conditions to normal temperature and normal humidity conditions.